1. Field of the Invention
The present invention relates generally to improvements in analog finite impulse response (FIR) based line-drivers, and more particularly to advantageous aspects of an analog-FIR-based filter embedded in a line-driver, by which wave-shaping, filtering and pre-equalization for the data stream are implemented.
2. Description of the Prior Art
According to the international standard ISO/IEC 8802-3 (ANSI/IEEE std 802-3, clause 14.3.1.2.1), any harmonics of a transmitted signal for an all-ones Manchester (return-to-zero) encoded signal have to be at least 27 dB below the fundamental, which in a 10 Base-T system is 10 MHz. For a 5 MHz signal (periodic "10" or "01" binary data), the high-frequency components of the signal have to be enhanced, thus increasing the transmission distance. This is typically called "pre-equalization." The amount of the pre-equalization should be such that the eye-diagram of the signal at the output of the twisted-pair model (ISO 8802.3) stays inside the voltage template shown in FIG. 14-9 of ISO 8802-3. In addition, it is required that the signal amplitude for a 100 .OMEGA. resistive load stay between 2.2 V to 2.8 V (i.e., 2.5 V.+-.12%).
FIG. 1 shows a known configuration, in which the preceding requirements are met by a four-pin driver requiring four off-chip resistors and a differential off-chip filter (ATT1MX10 data sheet, August 1997). It is possible to integrate the transmit filters onto the chip by using continuous-time techniques, such as a Gm-C filter technique. However, the cut-off frequencies of continuous-time filters move from their desired value due to process and temperature conditions. To maintain the cut-off frequency of the continuous-time filter, a process plus temperature tuning mechanism has to be employed. Furthermore, designing a continuous-time filter to accommodate a large output swing (5 V peak-to-peak.+-.12%) with a power supply of 5 V.+-.10% is very difficult and expensive. In addition, to accommodate the pre-equalization, a second parallel continuous-time filter is required, or at least the input-stage of the filter has to be replicated. This has the disadvantage of increasing the power consumption of the driver.